Fix/effective capabilities union

[HitanshiThakar]

Description

This change fixes a security issue in effective_capabilities where explicitly declared plugin capabilities could override capabilities implied by a plugin's safety level.

Changes include:

Update effective_capabilities to return the union of declared capabilities and safety-level implied capabilities.
Prevent plugins from downgrading their effective capabilities by declaring a narrower capability set.
Add regression coverage to verify that implied safety capabilities are always preserved when explicit capabilities are present.
Preserve existing behavior for plugins without explicitly declared capabilities.

Related Issues

Closes #1753

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

python -m pytest testing/backend/unit/test_capabilities_effective.py testing/backend/unit/test_capabilities.py testing/backend/unit/test_plugin_capabilities_inventory.py

Checklist

• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.